Current vehicle detection systems use antenna patterns to define transmitter locations on highway systems. The location accuracy of these systems is very poor and requires multiple large antennas to improve the performance. Receive amplitude measurement techniques can improve the performance somewhat but variations in signal strength between different vehicles have been measured at greater than 10 dB and the antenna patterns from these vehicles are not uniform. These problems have limited the use of "open road" toll systems and have resulted in the use of "barrier" toll systems that establish vehicle separation for location identification purposes.
Commonly assigned U.S. Pat. No. 5,307,349 describes a technique which will permit vehicles to be able to communicate with the highway infrastructure. One use for the system described in U.S. Pat. No. 5,307,349 is for the collection of tolls electronically on an open road. Commonly assigned U.S. Pat. No. 5,227,803 describes a method of measuring the location of the transmitters mounted in vehicles.